This invention relates to scroll apparatus and, more particularly, to improved scroll mechanisms for use in vacuum pumps.
Scroll mechanisms are known for operating compressors or vacuum pumps. In both cases the mechanism operates on the same basic principle in which two scroll components each comprising a flat disc on which are upstanding strips of uniform width defining a spiral (or scroll) type structure are caused to co-operate by placing the scroll components substantially co-axially together with the respective spiral type structures inter-engaging and allowing the second component to "orbit" relative to the first stationary component. In this way it is possible to trap a volume of air between the orbiting scroll and fixed scroll and urge the volume from one end of the respective spirals to the other. In a vacuum pump in particular, the volume enters via a pump inlet a space between the scrolls at the larger chamber end of the scrolls and exits via a pump outlet at the centre of the scrolls. The number of "turns" of the scroll structures determines the number of "stages" of the pump.
The second component is generally caused to "orbit" by allowing it to inter-engage with bearings positioned eccentrically on a rotating drive shaft which is itself driven by a motor. The second component is constrained from rotating by a system of free running cranks mounted between the flat disc of the scroll and the pump body or by a variety of other means, including "Oldham" couplings.
Scroll mechanism vacuum pumps are generally required to achieve a combination of a high inlet volumetric displacement coupled with a high compression ratio in use.
Known pumps operating on the basis of the above described principles generally have a single inlet or "start" and a single exhaust with a single passageway therebetween defined by the orbiting scrolls; this is referred to as a single start pump.
However, pumps are also known which possess two or more inlets and a corresponding number of exhausts and a corresponding number of passageways defined by the orbiting scrolls to link one inlet with one exhaust; these are referred to as multi-start pumps and generally allow a relatively high volumetric displacement in comparison with a single start pump but without achieving a high compression ratio.
With a single start pump, attainment of a high volumetric displacement can only be effected by making the overall size of the pump larger.
With multi start pumps, attainment of the required compression ratio can only be effected by providing an increased number of `turns` of the individual scroll structures and hence the number of pump stages; however, this obviously leads to an increase in the overall size of the pump which is generally undesirable on cost grounds at least.
The present invention is concerned with the provision of a new design of scroll pump apparatus which generally allows for a combination of high volume throughput and high compression ratio but with the benefit of achieving a relatively small size.